1. Field of the Invention
The present invention relates to an improved method for predicting the microstructure with which a certain cast iron melt will solidify. The invention also relates to an apparatus for carrying out the method.
2. Description of Related Art
WO86/01755 (incorporated by reference) discloses a method for producing compacted graphite cast iron by using thermal analysis. A sample is taken from a bath of molten cast iron and this sample is permitted to solidify during 0.5 to 10 minutes. The temperature is recorded simultaneously by two temperature responsive means, one of which is arranged in the centre of the sample and the other in the immediate vicinity of the vessel wall. So-called cooling curves representing temperature of the iron sample as a function of time are recorded for each of the two temperature responsive means. According to this document it is then possible to determine the necessary amount of structure-modifying agents that must be added to the melt in order to obtain the desired microstructure.
WO92/06809 (incorporated by reference) describes a specific method for evaluating the cooling curves obtained by the method of WO86/01755. According to this document, thermal analysis is carried out in a sample vessel coated with a material consuming the active form of the structure-modifying agent. This material may comprise oxides of Si, Mn, Fe, K and Na. An early plateau in the cooling curve recorded by a certain temperature-responsive means located near the vessel wall indicates that flake graphite has been formed due to interaction with the coating. It is then possible to determine whether any structure-modifying agent has to be added to the melt in order to obtain compacted graphite cast iron by using calibration data.
When casting compacted graphite cast iron in a commercial large-scale foundry, it is of utmost importance that accurate and reliable predictions of the micro-structure of the castings can be carried out. Sometimes cooling curves that are difficult to interpret are accepted as CGI despite the fact that some flaky graphite has been formed. In order to improve the accuracy of the evaluation, there is thus a need for alternative methods for evaluating cooling curves which methods can compensate for deviations from the normal appearance of the cooling curves.
Now, it has turned out that by studying heat transfer in a sample vessel containing a sample of molten cast iron, it is possible to carry out accurate predictions of the microstructure in which the molten cast iron sample will solidify. This method is also highly suitable for automatization by using a computer.
The term xe2x80x9ccooling curvexe2x80x9d as disclosed herein refers to graphs representing temperature as a function of time, which graphs have been recorded in the manner disclosed in WO86/01755 and WO92/06809.
The term xe2x80x9cheat generation curvexe2x80x9d as utilised herein relates to a graph showing the heat that is generated in a certain zone of a molten cast iron as a function of time. For the purposes of the present invention, the heat generation curves herein are determined for a zone located in the centre of a molten cast iron sample (the A zone), and in the periphery of a molten cast iron sample (the B zone), respectively. Below, methods for determining heat generation curves will be further described.
The term xe2x80x9csample vesselxe2x80x9d as disclosed herein, refers to a small sample container which, when used for thermal analysis, is filled with a sample of molten metal. The temperature of the molten metal is then recorded during solidification in a suitable way. Preferably the sample vessel is designed in the manner disclosed in WO86/01755, WO92/06809, WO91/13176 (incorporated by reference), WO96/23206 (incorporated by reference) or PCT/SE98/02122.
The term xe2x80x9csampling devicexe2x80x9d as disclosed herein, refers to a device comprising a sample vessel equipped with at least two temperature responsive means for thermal analysis, said means being intended to be immersed in the solidifying metal sample during analysis, and a means for filling the sample vessel with molten metal. The sample vessel is preferably equipped with said sensor in the manner disclosed in FIG. 2 in WO96/23206 or PCT/SE98/02122.
The term xe2x80x9cstructure-modifying agentxe2x80x9d as disclosed herein relates to compounds affecting the morphology of graphite present in the molten cast iron. Suitable compounds can be chosen from the group of magnesium and rare earth metals such as cerium, or mixtures of these compounds. The relationship between the concentration of structure-modifying agents in molten cast irons have already been discussed in the above cited documents WO92/06809 and WO86/01755.
The term xe2x80x9cCGIxe2x80x9d as disclosed herein refers to compacted graphite cast iron.
The term xe2x80x9cSGIxe2x80x9d as disclosed herein refers to spheroidal graphite cast iron.